1. Field of the Invention
This invention relates to optical isolators and particularly to an ultraviolet optical isolator which utilizes a KDP-isomorph for rotating the polarization of ultraviolet radiation in the wavelength range from about 190 to about 350 nanometers (nm).
2. Description of the Prior Art
Optical isolators using Faraday rotators have many uses in laser technology, such as for isolating stages of oscillator-amplifier systems and for discriminating against reverse-traveling waves in ring cavities. A Faraday rotator is a device that rotates the angle of polarization of light by using the Faraday effect. The Faraday effect is a magneto-optic effect wherein a magnetic field applied to a Faraday material causes the axis of polarization of the linearly polarized light to rotate. Faraday rotators have not been used extensively in the ultraviolet (UV) because of the limited availability of suitable Faraday materials. The two most important characteristics of a Faraday material are the ability of the Faraday material to rotate the polarization of light when the material is placed in a magnetic field, as indicated by the Verdet constant, V, of the Faraday material, and the optical transmission of the Faraday material at the wavelength of interest. The Verdet constant of a Faraday material is a function of wavelength. The optical transmission of the material is also dependent upon the wavelength. In general, Faraday materials with large Verdet constants have large absorption in the UV, while Faraday materials with high UV transmission have relatively small Verdet constants.
In the known prior art, only water and fused silica have been used in the UV as Faraday rotators. Water has been used in the UV as a Faraday material for XeCl and KrF lasers at 308 and 248 nanometers (nm), respectively. Water is suitable for large pulsed oscillator-amplifier systems, where a long path and strong pulsed magnetic fields can be used. However, the relatively small Verdet constant of water at these wavelengths makes it a less desirable Faraday material for small lasers where space may be limited. In addition, unless the water is kept extremely pure, its optical quality degrades severely. Sources of contamination of the water range from the leaching of chemicals from container walls to algae growth.
Fused silica has been used in the UV as a Faraday rotator with the KrF laser operating at 248 nm. The Verdet constant of various types of fused silica have been measured at visible wavelengths. The values for the various types of fused silica are in agreement with each other to within approximately 10%. However, the only reported measurement in the UV is for a specific type of fused silica, suprasil, at 253.7 nm. In general, values of the Verdet constant at UV wavelengths can be predicted, for a given material, by applying a simple scaling relation to measured values of the Verdet constant at visible wavelengths. This scaling relation has an inverse-square dependence on wavelength, so its value increases as the wavelength gets shorter.
For fused silica, from measured values of V in the visible, the scaling relation predicts Verdet constants of 1365 deg/T-m at 253.7 nm and 1430 deg/T-m at 248 nm. However, measured values of 1710 deg/T-m at 253.7 and 1920 deg/T-m at 248 nm have been reported for suprasil and for an unspecified type of fused silica, respectively. The discrepancy of 25-30% between the predicted and measured values is most likely due to the type and relative abundance of impurities present in the fused silica, and indicates that the UV Verdet constants of the various types of fused silica and even of different batches of the same type may be very sensitive to small changes in composition. It is well known that different types of fused silica have widely varying optical transmission in the UV due to impurities.
As indicated above, practical Faraday rotators for the UV would find applications with lasers operating at UV wavelengths. Of the two Faraday materials which previously have been utilized in the UV, fused silica has the disadvantages of widely varying composition and insufficient data on the UV Verdet constants to make it widely useful as a Faraday material in the UV, while water has a well-known, but relatively small UV Verdet constant and is easily contamined. Thus, there is a clear and compelling need for better UV Faraday rotating materials.